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package blok21_22BogsystemObserverPattern;import java.util.ArrayList;public class BogTitel implements Subject {	private String titel;	private int antal;	private ArrayList<Observer> observers = new ArrayList<Observer>();	private ArrayList<Kunde> kunder = new ArrayList<Kunde>();	public BogTitel(String titel, int antal) {		this.titel = titel;		this.antal = antal;	}	public void fire() {		for (int i = 0; i < observers.size(); i++) {			observers.get(i).update(this);		}	}	public String getTitel() {		return titel;	}	public int getAntal() {		return antal;	}	public void indkoebTilLager(int antal) { // OK		if (antal < 1) {			throw new RuntimeException("Antal skal være større end en");		}		this.antal += antal;	}	public void etKoeb(Kunde k) {		this.antal = antal - 1;		k.addBogTitel(this);		fire();	}	public void addObserver(Observer o) {		observers.add(o);	}	public void removeObserver(Observer o) {		if (observers.contains(o)) {			observers.remove(o);		}	}	public ArrayList<Kunde> getKunder() {		return kunder;	}	public void addKunde(Kunde k) {		kunder.add(k);		k.addBogTitel(this);	}}